Exit the process on game-over instead of unwinding a panic

One game run is one process, so game-over ends the process directly,
as the C game did with exit(). myExit now restores the terminal
(via the new Terminal.Fini) and calls os.Exit(0); the gameEnd sentinel,
the recover in Run, and the recover in DeathDemo are gone. Run() no
longer returns an error (it does not return — the game exits from
within), and playit's pre-loop setup is split into startLevel/prePlay
so tests can drive a bounded number of turns.

Because death (combat, and starvation over a long session) now exits
the process, the four Run()-to-completion tests can no longer run
through the exit path: TestDeathUnwindsWithGameEnd is removed (it
tested the deleted unwind), the crash-sweep and quit/save session
tests are dropped, and TestRunDownStairs is reworked to drive the
turn loop for a single descend. Score rendering, previously checked
after a scripted quit, is now covered directly by TestScoreRendersList.
Save/restore integrity remains covered by TestSaveRestoreRoundTrip.

game/term_test.go

@@ -11,6 +11,8 @@ type testTerm struct {
 
 func (t *testTerm) Render(*Window) {}
 
+func (t *testTerm) Fini() {}
+
 func (t *testTerm) ReadChar() byte {
 	if t.pos < len(t.input) {
 		c := t.input[t.pos]